Is there a reason why you don't want to read it in mobi format? You can use the Kindle app on almost any type of device.
– idiotprogrammerAug 30 '19 at 15:22

Because mobi is a proprietary binary format, it's going to be hard and messy to convert it into epub. Still harder for pdf. It's easier to convert from epub or docx to kindle or pdf than vice versa. I'm no fan of Amazon, but Kindle reading app is great! .
– idiotprogrammerAug 30 '19 at 15:31

I need to edit the file, so pdf/word files would be the ideal
– GJCAug 30 '19 at 16:01

You probably don't want to convert it to PDF, you will have problems that are unseen at this moment. Your best bet for edits is extracting all text, converting the text to markdown, and editing the markdown, unless there are a lot of images or tables in your source MOBI file. Once the file is in Markdown format use Pandoc to convert it to anything else, like EPUB, MOBI, etc. Pandoc is for Windows, Mac, and Linux and is free, well-supported, and has lots of features. Source: my day job is type-setting, hobby is converting PDFs to EPUB.
– BulrushSep 9 '19 at 16:59

Based on the comment thread, you want to edit the text of the book. The comment thread is going entirely in the wrong direction. The first point to understand is that .mobi is essentially just a .epub file wrapped up in Amazon's proprietary code. Kindle Unpack (Google it) is a free utility that will break open the mobi package so you can access the HTML files inside. There is an epub file in there too. Either edit the HTML files or just grab the epub file and do what you want to from there.

This is all assuming that you are not able to edit the mobi with Calibre. It probably has DRM protection since it's a dictionary. We're all dying to know which definition you're editing by the way 😂